With the recent increase in the amount of data communication, there is a growing need for mobile communication systems having higher frequency use efficiency. Thus, one-cell reuse cellular systems using the same frequency band in all cells are being studied.
An evolved universal terrestrial radio access (E-UTRA) system is one of the one-cell reuse cellular systems, and is being standardized mainly in 3rd Generation Partnership Project (3GPP). In the E-UTRA system, the use of an orthogonal frequency division multiple access (OFDMA) scheme is being studied as a downlink transmission scheme. Also, in the E-UTRA system, the use of a single carrier-frequency division multiple access (SC-FDMA) scheme as an uplink transmission scheme is being studied.
The OFDMA scheme is a scheme by which a user has access in resource block units divided by time and frequency by using an orthogonal frequency division multiple (OFDM) signal having excellent tolerance against multi-path fading. The OFDMA scheme is not suitable as a transmission scheme in which a transmission power limit is strict, since the OFDMA scheme has a high peak-to-average power ratio (PAPR) characteristic.
On the other hand, an SC-FDMA scheme is able to suppress the PAPR characteristic to be low in comparison with a multicarrier scheme such as OFDM and realize a wide coverage. Thus, the SC-FDMA scheme is suitable for uplink transmission (Non-Patent Document 1).
FIG. 14 is a schematic block diagram showing the configuration of a terminal device 100z when the SC-FDMA scheme is used for uplink transmission. As shown in FIG. 14, in the terminal device 100z using the SC-FDMA scheme, an encoding unit 1000 performs error correction coding on transmission data and a modulation unit 1001 performs modulation.
Next, a transmission signal modulated by the modulation unit 1001 is serial/parallel (S/P) converted in an S/P conversion unit 1002. Thereafter, a DFT (discrete Fourier transform) unit 1003 performs conversion from a time domain signal into a frequency domain signal.
The transmission signal converted into the frequency domain signal is mapped to subcarriers used for transmission by a subcarrier mapping unit 1004. At this time, the mapping is performed based on mapping information transmitted from a base station device. The mapping information is obtained as it is transmitted from the base station device to the terminal device 100z, received by a reception antenna 1011, converted from an analog signal into a digital signal by a radio unit 1012 and an A/D (analog/digital) conversion unit 1013, and demodulated by a reception unit 1014. In a mapping process, zero is inserted into subcarriers not used for transmission.
In the E-UTRA system, the use of “localized” mapping using continuous subcarriers is being studied. “Distributed” mapping using subcarriers separated by a uniform interval is also being studied in the E-UTRA system.
FIG. 15A is a diagram showing an example of localized mapping. FIG. 15B is a diagram showing an example of distributed mapping. In FIGS. 15A and 15B, the horizontal axis is the frequency. FIG. 15A shows the case where 72 subcarriers are arranged in the frequency direction. In FIG. 15A, the subcarriers are allocated to 6 users UA, UB, UC, UD, UE and UF (not shown). In this and other figures, subcarrier allocations are represented by different shading, where each shading represents a different user. Subcarriers allocated to the same user have the same shading.
FIG. 15A shows a localized arrangement in which the number of subcarriers constituting one subchannel is 12. Here, the 6 users (the users UA to UF) are frequency division multiplexed and are arranged in respective subcarriers. Specifically, the user UA is allocated to the 1st to 12th subcarriers in a frequency domain F901. The user UB is allocated to the 13th to 24th subcarriers in a frequency domain F902.
The user UC is allocated to the 25th to 36th subcarriers in a frequency domain F903. The user UD is allocated to the 37th to 48th subcarriers in a frequency domain F904. The user UE is allocated to the 49th to 60th subcarriers in a frequency domain F905. The user UF is allocated to 61st to 72nd subcarriers in a frequency domain F906.
FIG. 15B shows a distributed arrangement when the number of subcarriers constituting one subchannel is 8. FIG. 15B shows the case where 64 subcarriers are arranged in the frequency direction. Here, 8 users are frequency division multiplexed. FIG. 15B shows the case where the 64 subcarriers are arranged in the frequency direction. In FIG. 15B, the subcarriers are allocated to 8 users UA, UB, UC, UD, UE, UF, UG and UH.
Specifically, the 1st user is allocated to the 1st, 5th, 9th, 13th, 17th, 21st, 25th and 29th subcarriers. The 2nd user is allocated to the 2nd, 6th, 10th, 14th, 18th, 22nd, 26th and 30th subcarriers.
The 3rd user is allocated to the 3rd, 7th, 11th, 15th, 19th, 23rd, 27th and 31st subcarriers. The 4th user is allocated to the 4th, 8th, 12th, 16th, 20th, 24th, 28th and 32nd subcarriers.
The 5th user is allocated to the 33rd, 37th, 41st, 45th, 49th, 53rd, 57th and 61st subcarriers. The 6th user is allocated to the 34th, 38th, 42nd, 46th, 50th, 54th, 58th and 62nd subcarriers.
The 7th user is allocated to the 35th, 39th, 43rd, 47th, 51st, 55th, 59th and 63rd subcarriers. The 8th user is allocated to the 36th, 40th, 44th, 48th, 52nd, 56th, 60th and 64th subcarriers.
In the above-described mapping, the localized arrangement is suitable for obtaining a multiuser diversity gain, and the distributed arrangement is suitable for obtaining a frequency diversity gain.
Referring back to FIG. 14, a transmission signal is mapped onto subcarriers used for transmission by the subcarrier mapping unit 1004 of the terminal device 100z. The mapped transmission signal is input into an IFFT (inverse fast Fourier transform) unit 1005 and converted from a frequency domain signal into a time domain signal.
A P/S (parallel/serial) conversion unit 1006 converts the signal from a parallel signal to a serial signal.
A CP (cyclic prefix) insertion unit 1007 inserts a cyclic prefix. The cyclic prefix is a signal obtained by copying a rear portion of a symbol after IFFT.
Thereafter, a D/A (digital/analog) conversion unit 1008 performs conversion from a digital signal into an analog signal. A radio unit 1009 performs up-conversion into a radio frequency band signal, which is then transmitted from a transmission antenna 1010.
The transmitted signal generated as described above has the characteristic of a lower PAPR than a multicarrier signal.
FIG. 16 is a schematic block diagram showing the configuration of a conventional base station device 200z. The base station device 200z receives a signal transmitted from the terminal device 100z (FIG. 14).
The base station device 200z receives a signal of an SC-FDMA scheme. In the base station device 200z, a signal received by an antenna 2000 is converted into a frequency for A/D conversion by a radio unit 2001. Thereafter, an A/D conversion unit 2002 performs conversion from an analog signal into a digital signal.
Next, a synchronization unit 2003 establishes symbol synchronization. A cyclic prefix removal unit 2004 removes a cyclic prefix from every symbol. Thereafter, an S/P conversion unit 2005 performs conversion from a serial signal into a parallel signal. An FFT unit 2006 performs conversion from a time domain signal into a frequency domain signal.
A pilot signal for propagation channel estimation converted into the frequency domain signal is output to a propagation channel estimation unit 2007, and subjected to the propagation channel estimation. The pilot signal is a known signal transmitted with a data signal from the terminal device 100z. 
As shown in FIGS. 15A and 15B, in a signal received by the base station device 200z, signals transmitted from a plurality of terminal devices are frequency division multiplexed. In the signal received by the base station device 200z, subcarriers used for each terminal device are assembled by a subcarrier demapping unit 2008 based on mapping information determined in advance by a scheduling unit 2012. The mapping information is information indicating which terminal device uses which subcarrier.
Thereafter, an equalization unit 2009 performs an equalization process on received subcarriers assembled for each terminal device using a propagation channel estimation value. An IDFT (inverse discrete Fourier transformation) unit 2010 performs conversion from a frequency domain signal into a time domain signal. Thereafter, a demodulation and error correction decoding unit 2011 reproduces the transmission data from each terminal device and outputs the resultant data as reception data.
A pilot signal for reception level measurement is output from the FFT unit 2006 to the scheduling unit 2012. Based on the result of the reception level measurement using the pilot signal, the scheduling unit 2012 performs scheduling according to the propagation situation of each terminal device.
The mapping information determined in the scheduling unit 2012 is, for example, modulated in a transmission unit 2013. A D/A unit 2014 performs conversion from a digital signal into an analog signal. A radio unit 2015 performs up-conversion into a radio frequency, which is then transmitted from an antenna 2016 to each terminal device. The mapping information is used when a signal is transmitted in the next and subsequent frames at the transmitter.
As the uplink transmission scheme of the E-UTRA system, the above-described SC-FDMA scheme is most prominent. Like the OFDMA scheme, the SC-FDMA scheme is also a scheme by which a user has access in resource block units completely divided by time and frequency. By using the SC-FDMA scheme, it is possible to implement a one-cell reuse system which suppresses the PAPR characteristic to be low and realize a wide coverage.
However, in the current environment in which the shortage of frequency resources is accelerated with an increase of the number of users, it is necessary to increase the number of users capable of being accommodated and to further effectively use a frequency.
Non-Patent Document 1: 3GPP, TSG RAN WG1 on LTE, R1-050702, “DFT-spread OFDM with Pulse Shaping Filter in Frequency Domain in Evolved UTRA Uplink”